Cotler was a professor of law at McGill University and the director of its Human Rights Program from 1973 until his election as a Member of Parliament in 1999 for the Liberal Party of Canada. He has also been a visiting professor at Harvard Law School, a Woodrow Wilson Fellow at Yale Law School and is the recipient of eleven honorary doctorates. He was appointed in 1992 as an Officer of the Order of Canada. He is a past president of the Canadian Jewish Congress.

Cotler has served on the Standing Committee on Foreign Affairs and its sub-Committee on Human Rights and International Development, as well as on the Standing Committee on Justice and Human Rights. In 2000, he was appointed special advisor to the Minister of Foreign Affairs on the International Criminal Court.

Saad Eddin Ibrahim, an Egyptian democracy activist imprisoned by the Egyptian government, was represented by Cotler and acquitted in 2003. He acted as counsel to Maher Arar during part of Arar's imprisonment and supported demands for a public inquiry. He has also defended both Palestinians and Israelis against their own governments, and participated in a minor role in the Camp David peace agreement between Israel and Egypt.[4]

In 2017, Cotler was asked to join a panel of independent international experts designated by Luis Almagro, the Secretary General of the Organization of American States, to determine whether there was reasonable ground to believe that crimes against humanity have been committed in Venezuela[5].

One of the central challenges for Cotler during his time as Justice Minister was to address concerns about terrorism while guarding against arbitrary and unnecessary limits on rights. Part of his work in this regard included a review of Bill C-51, Canada's relatively recent Anti-Terrorism Act. The Anti-Terrorism Act has been criticized by some human rights groups and defense lawyers, as an unreasonable trade-off between security and freedom.[7] Cotler believed that the legislation did, in fact, strike a balance between rights and national security concerns, but understood that further consultation was necessary in reviewing the legislation. On February 21, 2005, Cotler spoke of the important work that Bill C-51 involved, and invited experts and other groups to continue dialogue to improve the legislation in the review process.[8]

Cotler presided over other legislative changes concerning national security. This included proposed changes to privacy legislation known as "Lawful Access" to give police and intelligence officers the tools to conduct surveillance of electronic communications for law enforcement and national security purposes.[9][10]

Cotler was re-elected to Parliament in the 2008 election to represent the Mount Royal riding in Quebec with 55% of the vote,[13] In January 2009, Cotler was named Special Counsel on Human Rights and International Justice for the Liberal Party, under Michael Ignatieff, and subsequently Critic for Human Rights. He was re-elected again in the 2011 election. In the 2011 election, Cotler fended off a serious challenge from former city councillor Saulie Zajdel, a longtime Liberal supporter running as a Conservative who lost by only 2,500 votes. It was only the third time that the Liberals have been seriously threatened in Mount Royal since initially winning it in 1940, and the closest that a centre-right party has come to winning anywhere in Montreal since 1993. In May 2011, Cotler was named Justice and Human Rights Critic by interim Liberal leader Bob Rae[citation needed]. Cotler also chaired the Inter-Parliamentary Group for Human Rights in Iran, the Inter-Parliamentary Group of Justice for Sergei Magnitsky, and the All-Party Save Darfur Coalition. [14]

On February 5, 2014, Cotler announced he was not running in the 42nd Canadian federal election. He said he would remain "active in public life, lecturing and writing on the issues of the day, advancing the causes of human rights and international justice, and advocating on behalf of political prisoners."[16]

Cotler was one of thirteen Canadians banned from traveling to Russia under retaliatory sanctions imposed by Russian President Vladimir Putin in March 2014.[17] He replied through his official Twitter feed, "I see my travel ban from Russia as a badge of honour, not a mark of exclusion."[17]

As Minister of Justice, Cotler tabled Canada's first-ever National Justice Initiative Against Racism, in parallel with the government's National Action Plan Against Racism. Cotler has worked with a group of international jurists to indict Iranian President Ahmadinejad for incitement to genocide under the UN Charter and the Genocide Convention.[20] Cotler released a petition in 2008 entitled The Danger of a Genocidal and Nuclear Iran: A Responsibility to Prevent Petition. He separated six categories of anti-Semitism and found thirteen indices of discrimination against Jews that characterizes the "new anti-Jewishness".[21]

At the request of Nelson Mandela's South African legal team, Cotler took on the role of "Canadian counsel" to Mandela in 1981, participating in anti-apartheid activities in Canada and advocating on Mandela's behalf.[26]